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111.
Mark B. Edlund Daniel R. Engstrom Laura D. Triplett Brenda Moraska Lafrancois Peter R. Leavitt 《Journal of Paleolimnology》2009,41(4):641-657
Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating
mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use
paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial
disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores
recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using 210Pb and 137Cs, and analyzed for historical changes (c. 1840–present) in sediment magnetic susceptibility, inorganic and organic matter
content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between
the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic
silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted
from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations
increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l−1 c. 1910 to ~60 μg l−1 since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did
not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream
environments was restricted to the latter half of the twentieth century.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue. 相似文献
112.
113.
Tatsuya Hayashi Yoshihiro Tanimura Yoshihiro Kuwahara Masao Ohno Mami Mampuku Rie Fujii Harutaka Sakai Toshiro Yamanaka Takeshi Maki Masao Uchida Wataru Yahagi Hideo Sakai 《Quaternary Research》2009,72(3):377-387
Variations in fossil diatom assemblages and their relationship with global and Indian monsoon climate changes for the last 600,000 yr were investigated using a core of ancient lake (Paleo-Kathmandu Lake) sediments drilled at the Kathmandu Basin, Nepal Himalaya. Chronological scales of the core were constructed by tuning pollen wet and dry index records to the SPECMAP δ18O stack record. Examinations of biogenic silica contents and fossil diatom assemblages revealed that variations in productivity and compositions of diatom assemblages were closely linked with global and Indian monsoon climate changes on glacial and interglacial time scales. When summer monsoonal rainfall increased during interglacials (interstadials), diatom productivity increased because of increased inputs of terrestrial nutrients into the lake. When summer monsoonal rainfall reduced and/or winter monsoonal aridification enhanced during glacials (stadials), productivity of the diatoms decreased and lake-level falling brought about changes in compositions of diatom assemblages. Monospecific assemblages by unique Cyclotella kathmanduensis and Puncticulata versiformis appeared during about 590 to 390 ka. This might be attributed to evolutionary fine-tuning of diatom assemblages to specific lake environmental conditions. Additionally, low-amplitude precessional variations in monsoon climate and less lake-level changes may have also allowed both species to dominate over the long periods. 相似文献
114.
Valentina Vanghi Andrea Borsato Silvia Frisia Daryl L. Howard Gertruida Gloy John Hellstrom Petra Bajo 《Sedimentology》2019,66(7):2661-2685
Synchrotron high‐resolution and micro‐X‐ray fluorescence elemental mapping of two coeval coralloid speleothems from Lamalunga Cave (Italy) are complemented with petrographic, morphological and microstratigraphic studies. The importance of these speleothems relies on their direct and indirect association with a complete Neanderthal skeleton (‘Altamura Man’) found inside the cave. The coralloids grew discontinuously between 64·6 ka and the Holocene and reveal exceptionally high concentrations of Mg, Sr and Si, particularly on convex surfaces, where evaporation is more intense. The incorporation of trace elements depends on several factors including location, shape and geometrical evolution during their growth, as well as climate and environmental parameters. This resulted in calcite precipitation with Sr compositions from 100 to 1200 ppm and an average concentration of 7000 ppm Mg. An unusually high Si content (up to 16%) is possibly derived from volcanic ash transported as particulate and in solution inside the cave. The most common fabrics observed consist of non‐fluorescent elongated columnar calcite forming clean isopachous bands and fluorescent fibre‐like crystals associated with laminated, lenticular bands high in Sr, Mg and Si. Variability in Sr, Mg and Si concentrations appears to induce fabric changes in the coralloids. Elongation and lattice distortion of the crystals was found to coincide with high Mg concentrations. The transition from compact elongated to open to fibre‐like, is here interpreted as due to high concentrations of Si and Sr, which are preferentially incorporated in the speleothem at crystal boundaries and intra‐laminae. It is here inferred that coralloid fabric changes and their elemental content potentially record local rainfall variations through time, with the clean compact calcite marking high infiltration and open fibre‐like and micrite fabrics recording dry periods. 相似文献
115.
High resolution transmission electron microscopy evaluation of silica glaze reveals new textures 总被引:1,自引:0,他引:1
We present the first nanoscale investigation of silica glaze. High resolution transmission electron microscopy of a rock coating from the Ashikule Basin, Tibetan Plateau, reveals the presence of spheroids composed predominantly of silicon and oxygen with diameters ranging from 20 nm to 70 nm. While silica glaze spheroids co‐exist with manganese‐rich rock varnish in the same sample, the different rock coatings are texturally and physically distinct at the nanoscale. These observations are consistent with a model of silica glaze formation starting with soluble aluminum‐silicon (Al‐Si) complexes [Al(OSi(OH)3)2+], mobilized with gentle wetting events such as dew or frost. The transition between complete and partial wetting on silica surfaces rests at about 20–70 nm for liquid droplets. Upon crossing this transition, a metastable wetting film would be ruptured, initiating formation of silica glaze through spheroid deposition. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
116.
Wim Clymans Eric Struyf An Van den Putte Christoph Langhans Zhengang Wang Gerard Govers 《地球表面变化过程与地形》2015,40(9):1171-1181
Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
117.
118.
This work describes the modification of silica gel with dimethylglyoxime, in order to prepare an effective sorbent for the preconcentration and determination of copper. The sorption capacity of dimethylglyoxime‐modified silica‐gel (DMGMS) was 71.37 mg g–1 and the optimum pH for the quantitative recovery of copper was found to be 5.0. The optimum flow rate, sorbent amount, and sample volume were 1 mL min–1, 300 mg, and 50 mL, respectively. 10 mL of 0.1 mol L–1 HCl was the most suitable eluent. The detection limit of copper was 6.0 ng mL–1. The recommended method, for the determination of copper, is simple and reliable, without any notable matrix effect and can be successfully applied to environmental water samples. Copper recovery in the range from 99–100% was obtained from seawater and thermal spring water using this method. The method was applied to standard reference materials, NIST‐1515 (apple leaves) and NIST‐1643e (simulated fresh water), for the determination of copper and the results were in good agreement with certified values. 相似文献
119.
Sedimentary records of eutrophication in the Changjiang Estuary upwelling area over last 100 a 总被引:3,自引:1,他引:2
FENG Xuwen JING Xianglong YU Xiaoguo LI Hongliang CHEN Jianfang QIAN Jiangchu 《海洋学报(英文版)》2008,27(6):49-61
The upwelling area in the Changjiang Estuary was selected to collect the core, where the red tide occurred frequently and hypoxic existed. The total organic carbon (TOC), total nitrogen (TN), biogenic silica (BSi) and stable organic carbon isotopic ratios(δ13 Corg) were determined on the 210pb-dated sediment core. The concentrations of TOC, TN, BSi as well as their sedimentation fluxes have in- creased to some extent since the 1970s. TOC and TN fluxes increased about 45%, 36% respectively. The average δ13 Coorg value in the core was -23.67 ×10^-3 which remained nearly constant before the 20 century. The δ13 Corg values increased after the 1900s, two marked increases were observed from the 1950s and the 1970s. A simple δ13 Cors model was used to estimate the contribution of terrigenous and marine organic matter inputs for the sediment, which indicated the increase in accumulation since the 1970s has been almost exclusively marine. The increasing of marine organic matter accumulation (TOC, TN and BSi) was corresponding with the increasing of fertilizer consumption and the NO3-N budgets from the Changjiang River. The riverine runoff of fertilizers and nutrients stimulated the algae blooming. Enhanced primary production resulted in an enrichment of organic matter in the sediment. These data support the hypothesis that anthropogenic nutrient loading has been a significant factor on the eutrophication in the Changjiang Estuary. 相似文献
120.
地幔富硅交代与大陆岩石圈的演化 总被引:4,自引:0,他引:4
富硅交代是弧下地幔中熔体岩石相互作用的主要表现形式 ,是造成古老克拉通陆下岩石圈地幔富硅的主要机制。在弧下地幔捕掳体中 ,橄榄岩被来自俯冲洋壳物质部分熔融生成的含水富硅熔体交代后 ,斜方辉石含量的增加使全岩富集SiO2 ,斜方辉石显示异常低的Al2 O3和Cr2 O3,微量元素上表现为强烈富集LILE ,强烈亏损Nb ,Ta和Ti。在古老克拉通地幔岩样品中 ,方辉橄榄岩具过剩的斜方辉石 ,橄榄石的Ni含量与斜方辉石的组成含量成正比 ,而和橄榄石的x(Mg) /x(Mg +Fe2 +)值没有正比关系 ,被解释为亏损的地幔橄榄岩和来自俯冲板片的富硅熔体相互作用的结果。熔体岩石相互作用最终导致了陆下岩石圈地幔富集SiO2 ,这种被含水富硅熔体改造后的地幔岩石的部分熔融可能是造成陆壳富硅富镁的主要原因。含水富硅熔体对岩石圈地幔的影响程度也可能是大陆岩石圈增生或裂解、增厚或减薄的关键因素之一。 相似文献